Amazon Total War is a partial conversion for RTW v1.5 or BI v1.6. There are separate versions of the game for each executable. It uses the mod:switch option, so it will not interfere with any of your original game files.
ATW adds three new Amazon factions to the game: Amazonia, New Amazonia, and the Gorgons. These belong to a new culture, that of the Amazons, with new units, generals, retinue members, and traits just for them. My inspiration for this was the Amazons of legend, and I have tried to keep everything in the mod in the spirit of those legends. So just as legend has it, the Amazons are outstanding archers and horsewomen. They also fight in the standard greek hoplite style, as well as having some troops using the sagaris, an axe that was a signature weapon of the Amazons (and where calling a woman an "old battle-axe" comes from).
But Amazons and are not all that is different in ATW. There are 29 new territories, new units and skins for nearly every faction, and much more.